Sources of Information About Earth’s Interior
- Mined rocks give us the samples for direct observation. We get the rocks directly from below the earth surface. However, there is a limit to mining. So far scientist have drilled upto 12 km.
- Volcanic eruption forms another source of Obtaining direct information. As and when the molten material (magma) is thrown onto the surface of the earth, during volcanic eruption it becomes available for laboratory analysis. However, it is difficult to ascertain the depth of the source of such magma. Further it also hints towards the presence of molten/ semi molten layer (magma chamber) within the solid Earth.
- Rate of change of temperature, pressure and density indirectly help us to ascertain the structure, composition and thickness of the earth.
- The observable density of the outer crust is around 3.0-3.5 and By Newton’s gravitational law we know that avg. density of the earth is somewhere near 5.5. This means that the density successive layers must increase and must be more than 5.5. This further proves that the earth is made up of different materials whose density increases with increasing depth. (SiAl > SiMa > NiFe)
- Temperature rise per km is around 25 degrees Celsius. If we believe this to be true, then temperature at 2900 km depth would be around 72500 degrees Celsius which is way beyond the melting point 1200 degree Celsius. At such a high temperature the earth would have melted. This proves that even though temperature rises with depth inside Earth, but the rate of temperature rise decreases with increasing depth.
- Another source of information are meteors that at times reach the earth. However, it may be noted that the material that becomes available for analysis from meteors, is not from the interior of the earth. The material and the structure observed in the meteors are like that of the earth. They are solid bodies developed out of materials same as, or like our planet. Hence, this becomes yet another source of information about the interior of the earth.
Meteoroids: Before the small bit of comet or asteroid enter Earth’s atmosphere, it floats through interplanetary space and is called a meteoroid.
Meteors: Meteoroids which enter the atmosphere and burn up completely. Meteorites: Those meteoroids which reach the earth’s surface.- Gravitation force (g) is different at different latitudes on the surface (greater near the poles and less at the equator because of higher distance from the centre at the equator). The uneven distribution of mass of material within the earth influences this value. These readings differ from the expected values. Such a difference is called gravity anomaly. Gravity anomalies give us information about the distribution of mass of the material in the crust of the earth.
- Seismic waves travel differently in different mediums. Their direction and velocity change upon entering new mediums. Further the shadow zones of different seismic waves also differ. This helps us to understand the compositions of the Earth’s interior.
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